Dead Space Calculator

Respiratory dead space is the part of each breath that does not eliminate carbon dioxide. Anatomical dead space is air in conducting passages such as the nose, trachea, and bronchi, where gas exchange does not occur. Alveolar dead space is ventilated air that reaches alveoli but does not effectively exchange gas with blood. Physiological dead space combines both components and therefore reflects the total ineffective portion of tidal ventilation. This calculator applies the Bohr equation: physiological dead space divided by tidal volume equals arterial carbon dioxide pressure minus mixed expired carbon dioxide pressure, divided by arterial carbon dioxide pressure. Multiplying that fraction by tidal volume gives dead-space volume in milliliters. The calculation assumes arterial CO2 reasonably represents alveolar CO2 and requires a true mixed-expired sample. End-tidal CO2 is not interchangeable with mixed-expired CO2, especially when ventilation and perfusion are uneven. A simple anatomical reference is also shown using approximately 2.2 milliliters per kilogram of body weight. That estimate is useful for context but is not a patient-specific measurement. Airway devices, posture, age, lung volume, and breathing pattern can alter anatomical dead space. Physiological dead space may rise with pulmonary embolism, emphysema, low cardiac output, excessive positive pressure, or other ventilation-perfusion abnormalities. Dead-space fraction is often more informative than volume alone because it relates wasted ventilation to the size of each breath. Interpretation varies with clinical setting, sampling method, mechanical ventilation, and acid-base state. Trends obtained with the same equipment and technique may be more useful than a single isolated value. A surprising result should prompt confirmation of units, sample timing, equipment dead space, and whether the expired measurement truly represents the whole breath. This educational tool cannot diagnose lung disease or determine ventilator settings. Clinicians integrate blood gases, capnography, respiratory mechanics, imaging, hemodynamics, and the patient's condition. Anyone with severe breathlessness, blue discoloration, confusion, chest pain, or rapidly worsening respiratory symptoms needs urgent medical evaluation rather than an online estimate.